A typical commonly used battery, such as a lead acid battery cell, includes a positive electrode, a negative electrode, and a porous separator between the two electrodes. The electrodes and the separator are positioned in a battery container where they are surrounded by an electrolyte solution. During normal operation of the battery, the electrodes undergo an oxidation-reduction (redox) reaction. This reaction effects a transfer of ions between the electrodes. The flow of ions is manifested as a charge flow, which appears as the battery current.
The basic process for charging a typical battery, such as a lead acid battery, can be divided into three stages: an efficient charging stage, a mixed charging stage, and an overcharge or gassing stage. During the efficient stage of charging a lead acid battery, the predominant charging reaction is the transformation of PbS.sub.4 into Pb and SO.sub.2. During this period, the charge acceptance, defined as the ratio of the current transformed into electrochemical storage in the battery to the charging current provided from the battery charger, is nearly 100%. The efficient stage of the battery charging is normally over when the battery's state of charge reaches 70-80% of its normal fully charged level.
During the mixed stage of battery charging, two processes proceed simultaneously. The main charging processes continue, while, simultaneously, water electrolysis processes begin and gradually increase. As charging continues during the mixed charging stage, charge acceptance by the battery is gradually and continuously reduced. Once the battery cell voltage shows no further increase, the battery may be considered fully charged. At this point, the mixed charging stage is terminated.
If charging of the battery continues after the battery is considered fully charged, the battery may become overcharged, resulting in gassing of the battery. As charging continues after the battery cells are fully charged, water decomposition and self discharge processes proceed in the battery. The continuing water electrolysis process causes a rather large part of the energy loss in lead acid batteries.
Water electrolysis can be caused by excessively high charging current levels, as well as by continuing to charge a battery after full charge is reached. The level of charge current flowing into a battery is typically controlled by the battery charging system. Care must be taken so that the battery charging system does not produce such a high current level that the natural redox reaction capability of the battery cell electrodes is exceeded. Such a high charging current results in electrolysis of the aqueous electrolyte.
Electrolysis of the aqueous electrolyte away from the electrodes, resulting from either overcharging or excessive charging currents, releases gaseous oxygen and hydrogen from the electrolyte instead of soluble oxygen and hydrogen ions. This produces gas bubbles close to the electrode surfaces. The gas bubbles block the flow of ions to the electrode surfaces. Therefore, the effective electrode surface area is reduced. This, in turn, diminishes the current handling capability of the battery cell.
Continued charging of a battery which has entered the gassing stage can result in pressure buildup within the battery container. If the pressure builds up beyond the seal capability of the battery container, hydrogen and oxygen gasses will leak from the battery container. The resulting fluid loss will further decrease the current handling capability of the battery cell.
To prevent overcharging and gassing, a typical battery charger will terminate charging when a battery becomes fully charged, before the onset of overcharge conditions. The onset of the overcharging condition can be detected by monitoring the battery voltage. FIG. 1 shows cell voltage versus voltage capacity returned, as a percentage of the previous charge on the battery, for different charging rates. As can be seen, cell voltage rises sharply just before the battery cell voltage levels off at its fully charged level. This sharp rise in voltage indicates the beginning of overcharge reactions. Thus, a battery charger can detect the voltage rise and stop charging in order to avoid overcharge and gassing of the battery cell.
FIG. 1 also illustrates another interesting charging characteristic of many batteries. The lines 41-44 in FIG. 1 indicate battery capacity returned for different charging rates. The charging rates are presented as a fraction of C which is the rated amp-hour capacity of the battery cell. For example, line 41 indicates battery capacity returned for a battery charged at C/5 amps/hour. It is apparent from FIG. 1, that, for the onset of overcharge reactions to coincide with 100% return of battery cell capacity, the charge rate must typically be less than C/100. At higher charge rates, premature gassing will occur and charging will have to be terminated before the full battery cell capacity is reached.
Many battery powered systems require series connected strings of batteries to achieve desired operating voltage levels. For example, electric vehicles (EVs) employ series connected battery strings with total bus voltages in the range of 300 volts to realize the main motive force of the vehicle. Battery life, and the corresponding requirement to increase the time between rechargings, is one of the major factors presently limiting the realization of economically viable EVs. One method of increasing battery life is to improve the systems and methods for charging the series connected battery strings which power the EVs. The key to increasing battery life of a series connected string of cells is equalization of the charge on the individual cells which make up the series connected battery string. Maintenance of cells at an equalized charge level is critical for enhanced battery life. Charge equalization enhances uniformity of the battery cells, improving the life of the individual cells, and improving the life of the battery string as a whole.
During charging of a battery composed of a series connected string of cells, individual cells will often become charged to different levels, and will obtain correspondingly different cell terminal voltages. These potentially large non-uniformities among the battery cells are due to differences in cell chemistry, temperature gradients along the string of battery cells which effect their charging and discharging rates, and normal differences which occur during repeated cycles of cell charge and discharge. When a battery string is charged as a whole, individual battery cells are charged serially. Thus, some cells will reach full charge before other cells, due to the fact that some cells will simply charge more rapidly than others. Significantly, some cells will reach full charge before the overall battery terminal voltage reaches its nominal level. Charging of the battery string beyond this point can, therefore, lead to overcharging of a subset of the battery string cells. If these cells are charged beyond the onset of the gassing stage, there can be significant degradation of the life of both the individual battery cells, and the battery string as a whole. There is also the potential for damage to the overcharged battery cells. If, on the other hand, the charging process is stopped when only some of the battery cells are fully charged, the capacity of the battery string as a whole will not be fully utilized. Moreover, this would increase the risk of the undercharged cells going into polarity reversal during a deep discharge of the battery string.
Several schemes and algorithms for charging series connected strings of battery cells have been proposed. One method of charging a series connected string of battery cells which has been proposed involves multi-step constant current charging. In accordance with this method, the maximum current that the battery charger can deliver is applied to the battery string at the initiation of charging, when the battery cells are at a low state of charge. As the state of charge of the battery string begins to build up, the charging current is gradually reduced in steps. Periodic rest, or cooling periods are incorporated into the battery charging process. These rest periods are used to reduce temperature differences among the cells which make up the battery string. This will tend to equalize the charge on each of the individual cells making up the battery string. Finally, equalization charging is applied at low current levels to the battery string to improve the battery capacity.
A fast charging algorithm for the charging of a series connected string of battery cells has also been proposed. This fast charging method includes three operational modes: an active-charge mode, an active-discharge mode, and an inactive rest mode. During the active-charge mode, positive pulses are applied to the battery string to supply energy to the battery. During the active-discharge mode, a sharp depolarization pulse, of much shorter duration than the active-charge mode pulses, is applied to the battery string to position the electrolyte ions in the battery cells away from the battery electrode plates. During the inactive rest mode, a stabilization or rest period is used to position the electrolyte ions at an optimum distance away from the electrode plate surfaces. Throughout this process, battery cell monitoring is used to optimize the charging algorithm.
Other schemes and algorithms for charging a series connected string of battery cells have been suggested. Most of these schemes, including those discussed above, deal with charging the entire string of battery cells as a whole. Thus, charging is provided to the entire string of battery cells at the terminals of the battery cell string. Using these whole string charging methods, the equalization of charge on individual cells which make up the battery string cannot be easily achieved.
To prevent the adverse effects of unequalized charging of battery cells, individual cells, or modules, need to be maintained at an equalized charge level during the charging process. Methods to achieve such charge equalization have been proposed which involve charging a string of battery cells by monitoring and recharging the battery cells on a single cell basis. By these methods, it is possible to maintain each battery cell at its optimal operating point, and to thereby maximize the life of the battery cell string as a whole. Typically, however, this is an expensive and inefficient approach to charging a series connected string of batteries.
A similar approach to equalization of charge across a series connected string of battery cells involves the use of an individual cell voltage equalizer across each battery cell in the string. The individual cell voltage equalizers are voltage controlled current shunts which divert current away from the individual cells during trickle charging. Trickle charging is typically constant voltage, low current charging, which is applied to a battery or string of battery cells as the battery or battery string approaches the fully charged state. The individual cell voltage equalizer scheme prevents fully charged cells from being overcharged, while undercharged cells can still be trickle charged. This scheme can be implemented using low power DC-to-DC converters across each battery cell. However, this scheme will typically be relatively expensive, due to the additional hardware and control which must be associated with each battery cell in the battery string.
A compromise to individual charging of each battery cell in a large string of cells is based on the recognition that battery life, under a normal operating cycle, tends to degrade almost exponentially as the battery string length is increased. Thus, a reasonable effort may be made to extend battery life by attempting to equalize small strings of battery cells within the main string of battery cells, instead of attempting to equalize the charge on each single battery cell. For example, in electric vehicle applications, the nominal battery voltage of 300 volts requires 25 stacks of 12 volts each. Each stack includes several battery cells connected in series. The present approach to charging the series connected battery string involves monitoring each 12 volt stack to detect the onset of the gassing stage. At this point, fast charging of the battery string may be stopped. Charge equalization may then be attempted on the entire series connected battery string by applying a smaller charge current to the string. This process, however, is not desirable because, even at the smaller current levels, the battery charger continues to pump charge into overcharged battery cells. As described above, an alternative approach to charge equalization in this case would be to use 25 battery charger converters, one for each stack. Thus, the charge rates for each of the stacks could be independently controlled to better equalize the final cell voltage. However, the increased hardware and control requirements would make this approach too expensive.